Stereomicroscopes are used to manipulate objects under visual observation and/or to make fine object details visible. The object manipulation preferably takes place under low magnification and requires good 3D reproduction. For detail recognition, rapid switching to high magnifications with high resolution is desired without change of instrument.
Stereomicroscopes provide two views of the object at various observation angles which are perceived by the viewer as a three dimensional image of the object. If the angle between the two observation directions is unusually large, the object appears spatially distorted.
Numerous descriptions of the telescope type of stereomicroscopes appear in the literature: see “Optical Designs for Stereomicroscopes”, K-P. Zimmer, in International Optical Design Conference 1998, Proceedings of SPIE, Vol. 3482, pages 690-697 (1998). The U.S. Pat. No. 6,816,321 discloses an afocal zoom system for high performance stereomicroscopes with which zoom factors z (ratio of maximum to minimum zoom magnification) of more than 15 can be achieved.
Stereomicroscopes with such a design include—apart from optional bolt-on modules—a main objective, which images the object at infinity, two downstream parallel telescopes for varying the magnification and two observation units (binocular tubes) comprising a tube lens, inverting system and eyepiece for visual observation with both eyes. The telescopes can be designed as Galilean telescopes for step-by-step magnification selection or as afocal zoom systems for continuous magnification selection. According to the prior art, two identical telescopes are arranged symmetrically to a plane of symmetry of the device, wherein the plane of symmetry divides the object symmetrically into a right and a left half. The distance between the telescope axes is referred to as the stereo basis. The numerical aperture of this microscope is given by the semi-diameter of the entrance pupil of the telescope or spy glass divided by the focal length of the main objective.
The numerical aperture of a microscope of this type is therefore limited in the prior art. In order to increase the numerical aperture it is known to expand the entrance pupil diameters of the telescopes which, because of the arrangement of the two telescopes next to each other, results in an expansion in the stereo basis and therefore in the drawback of large equipment dimensions, or to shorten the focal length of the main objective, thereby disadvantageously reducing the working distance and increasing the power required of the main objective excessively. In both cases the angle between the directions of observation is expanded, resulting in, or increasing, spatial distortion.
U.S. Pat. No. 5,603,687 describes an asymmetrical stereooptic endoscope, in which two objective systems with different diameters of the entrance pupils are arranged parallel next to each other. Both objectives produce images of the object on a sensor surface via light conductors or light fibers. From these CCD sensors for example, the image data are transmitted after digital processing to a monitor, that is to say they can be spacially perceived for example with a stereomonitor. It is stated that despite varying diameters of the two endoscopic channels the viewer perceives a stereoscopic image with a resolution and a brightness, as they result from the channel of larger diameter. The second channel of smaller diameter primarily serves to produce a stereoscopic vision or impression.
The conditions in the case of a stereomicroscope of the telescope type of the design as described above are in principle different than in the case of an endoscope in accordance with U.S. Pat. No. 5,603,687. Firstly, the viewing of the object takes place as a rule (at least also) directly with the eyes, without prior digital processing. Such digital processing will or can be used, if additionally documentation is to be made via connected cameras. It is not clear from the US document mentioned, how in the case of the embodiment described therein an object can be viewed directly visually. Furthermore, the projection onto a sensor surface (fixed focus) limits the depth of field of the display since the accommodation capacity of the eyes is out of action.
The magnification of an endoscope depends on the object distance. At high magnifications the object distance is normally minimal. In this case the overlap range of the fields of view of the two objectives being arranged next to one another is minimal. Therefore, stereoscopic viewing, which is only possible in the overlap range, is reduced in this case. At low magnifications however the overlap is large, but the numeric aperture is small, which results in high depth of field. Hence it follows that the image definition or quality of 3D objects only reduces slowly with the distance to the focus plane. This circumstance favours the merging of the two fields into a spatial image, in particular if the object depth is less than the depth of field.
A main component of a stereomicroscope of the type described is the telescope systems (discrete magnification changer or continuous zoom) in the two stereo channels. Telescope systems are not common in endoscopy. In the US document mentioned, therefore, a variation of the display scale or reproduction scale is not discussed.
For stereoscopic viewing the depth of field is important. In contrast to the stereoendoscope described above high power stereomicroscopes of the telescope type advantageously use the accommodation capacity of the eyes. A magnification variation takes place without changing the focusing of the equipment. There is no difference in the object clip between the right and the left field over the whole magnification range. The numeric aperture and thus the resolution of the stereomicroscope are adapted to the magnification and prevent empty magnifications. At high magnifications the depth of field is very small, in many cases smaller than the object depth in such arrangements. The image quality of 3D objects therefore considerably decreases with the distance to the focus plane. Thus, it cannot be assumed that the merging of the fields to a spatial image observed with a stereoendoscope under typically low magnification and high depth of field can be transferred to the conditions, which exist with a high power microscope in particular at high magnifications, if the stereoscopic channels due to different apertures produce images of different resolution and depth of field.
A further, not to be neglected, criterion is that of the image brightness, which is different in the case of the US document mentioned, due to the different entrance pupil diameters of the endoscopic channels. Here the digital processing of images has the advantage that both fields can be shown equally brightly on the monitor after corresponding correction. Such corrections are not possible in the case of direct visual viewing, as is the case with stereomicroscopes.
Furthermore, it would be detrimental with an arrangement discussed above, if the higher power of one of the stereoscopic channels could not be used by a user having eyes of different capability, if the stereoscopic channel of higher power was assigned to the eye of lower capability.
U.S. Pat. No. 3,655,259 describes a somatic microscope, which is to be used as an endoscope. This microscope has been developed as a stereomicroscope from the Greenough type. The two stereo channels are arranged at a given opening angle to one another and in each case possess their own objectives, which are designed here as mini lenses, rod lenses or as final sections of a glass fibre. The underlying problem with this somatic microscope of this document is due to the fact that with the use of two objectives these cannot be placed arbitrarily close to each other, since a lens combination is selected as the objective and the use of a single objective lens is not possible due to increasing spherical aberrations, in particular if high magnification is required. The object of the document mentioned is therefore to find an arrangement which permits minimum endoscope diameter at high magnification.
U.S. Pat. No. 4,862,873 describes a further stereoendoscope which comprises two channels arranged parallel to each other, wherein one of the channels is to be used for lighting and the other is to be used for observation respectively. In order to produce a stereoscopic image impression the two channels are switched over by a motorized prism 30 times per second for example.
German Patent DE 102 25 192 B4 describes an arrangement for an objective for stereomicroscopes of the telescope type as well as a corresponding stereomicroscope. The design, functional mode as well as the interrelationships of magnification, resolution power and vignetting, thereof are incorporated herein by reference.